In addition to numerous other sweeping changes to the patent laws, the American Invents Act (AIA) eliminates a challenge to validity of a patent based on failure to comply with the requirement that the inventor disclose the “best mode” contemplated for carrying out the invention. The Act does not eliminate the best mode requirement altogether—the Patent Office may reject an application for failure to satisfy the requirement—but it removes the applicant’s failure to comply as a “basis on which any claim of a patent may be canceled or held invalid or otherwise unenforceable” for actions commenced on or after September 16, 2011.
Best Mode Requirement
Pursuant to 35 U.S.C. § 112, which has not been amended under the AIA, there are three distinct requirements for disclosing an invention in a patent application: (1) the “written description” requirement – an adequate “written description” of the invention; (2) the “enablement” requirement – a description of the “manner and process of making and using [the invention]” in such a manner as to “enable any person skilled in the art” to make and use the invention; and (3) the “best mode” requirement – that the patent specification disclose “the best mode contemplated by the inventor of carrying out his invention.” 
Because the best mode disclosure is a statutory requirement for patentability, the invalidity of a patent for failure to comply with this requirement has long been a valid defense in an action involving infringement or validity of a patent. The AIA has now removed this defense.
Rationale for Eliminating the Best Mode Defense
The best mode requirement has been the subject of extensive criticism, primarily relating to (1) its reliance on the inventor’s subjective state of mind  and (2) its uniqueness to the U.S. patent system. Indeed, elimination of the requirement altogether had been urged historically by commentators and by stakeholders during consideration of patent reform legislation by Congress. As enacted, however, the AIA merely removes violation of the best mode requirement as a basis for an assertion of patent invalidity or unenforceability in litigation or for initiation of a post-grant review proceeding.
Language eliminating the best mode requirement as a basis for patent invalidity, but retaining the statutory requirement for purposes of obtaining a patent, first was introduced during debate on the Patent Reform Act of 2007, H.R. 1908, in amendments offered by Representative Mike Pence. In remarks concerning his amendments, Rep. Pence stated that the intent of retaining “best mode as a specifications requirement for obtaining a patent [was] to maintain in the law the idea that patent applicants should provide extensive disclosure to the public about an invention.” Importantly, it remains to be seen what the penalty is for not disclosing the best mode in the application without the threat of patent invalidity. One approach by litigants could be to attempt to prove inequitable conduct for intentional failure to disclosure the best mode; though with the increased proof threshold, this may be a difficult road.
The Pence amendments focused instead on removing best mode from litigation and post-grant review proceedings because the requirement “has become a vehicle for lawsuit abuse. . . . Increasingly in patent litigation defendants have put forth best mode as a defense and a reason to find patents unenforceable. It becomes virtually a satellite piece of litigation in and of itself, detracts from the actual issue of infringement, and literally costs American inventors millions in legal fees.”
Section 15 of the Act
Guided by the foregoing concerns, Congress enacted Section 15 of the Act, amending 35 U.S.C. § 282. Before that enactment, Section 282 provided that among the “defenses in any action involving the validity or infringement of a patent” shall be the following: “Invalidity of the patent or any claim in suit for failure to comply with any requirement of sections 112 or 251 of this title[.]”
The Act amends Section 282 by striking the aforementioned language and providing instead that such defenses shall include “[i]nvalidity of the patent or any claim in suit for failure to comply with” any requirement of Section 251 or “any requirement of section 112, except that the failure to disclose the best mode shall not be a basis on which any claim of a patent may be canceled or held invalid or otherwise unenforceable[.]” The Act further provides that Section 15 shall be effective immediately upon enactment and shall apply to all proceedings commenced on or after that date.  As a result, a litigant challenging the validity or enforceability of a patent—whether as a defendant in a patent infringement action or as a plaintiff in a declaratory judgment action under 28 U.S.C. § 2201—cannot rely on failure to satisfy the best mode requirement in any action filed on or after September 16, 2011. ____________________________________